A host is what the souls call the body they occupy. The hosts for the souls must have something like a brain and be larger than the souls. The souls replace the consciousness of the host body after insertion and become the ones controlling it. The exception to this is resistant hosts, unique to planet Earth. For a host to be resistant, they had to be an adult and aware of what would happen to them before insertion. Eventually, only infants and children were offered as hosts due to the risk of adults being too strong to subdue. Souls have no natural lifespan or aging process, although their host bodies do. When a host nears death, the soul is transferred to another. The hosts continue to eat what they always have and essentially their bodies function the same way.
The souls never choose to assimilate a planet until they can maintain the hosts perfectly. In order to do this they abduct members of the target species to learn of their physiology. They used their Spider hosts (the most intelligent) to invade the Earth at first.